Barcelona - World champion Lewis Hamilton had to pull out of the opening day of the second pre-season test in Barcelona due to a fever after completing just 11 laps.
The Englishman was replaced by Mercedes test driver Pascal Wehrlein, who had started the day testing Force India's car from the 2014 season.
Mercedes logged over 500 laps over four days in the first test of the season in Jerez and team principal Paddy Lowe explained they didn't call in Hamilton's teammate Nico Rosberg for the rest of the day so as to not overtax the German's workload with last season's runner-up in the world championship due to drive on Friday.
"For the programme we have for today, it is better to use a test driver," he told Sky Sports.
"It's a tough programme and we don't want to over-use Nico Rosberg."
Sauber's quick pace from the first test continued as Brazilian Felipe Nasr, who topped the time sheets on day three in Jerez, went fastest in 1min 27.307 ahead of Lotus' Pastor Maldonado and Daniel Ricciardo in the Red Bull.
Susie Wolff made the most of her morning testing the Williams as the Scotswoman racked up more laps than anyone else with 77.
However, there were more troubles for McLaren as after a promising start, their day ended after just 21 laps from Jenson Button due to a faulty seal meaning a power unit change is needed before they can try again with Fernando Alonso at the wheel on Friday.
Results from the morning session:
1. Felipe Nasr (BRA/Sauber) 1:27.307 (56 laps)
2. Daniel Ricciardo (AUS/Red Bull) 1:27.363 (18 laps)
3. Pastor Maldonado (VEN/Lotus) 1:27.510 (48)
4.Max Verstappen (NED/Toro Rosso) 1:27.951 (35)
5. Kimi Raikkonen (FIN/Ferrari) 1:28.064 (29)
6. Jenson Button (GBR/McLaren-Honda) 1:28.182 (21)
7. Pascal Wehrlein (GER/Force India) 1:28.329 (32)
8. Pascal Wehrlein (GER/Mercedes) 1:29.286 (6 laps)
9. Susie Wolff (GBR/Williams) 1:29.440 (77 laps)
10. Lewis Hamilton (GBR/Mercedes) 1:30.429 (11)